1. Field of the Invention
The present invention relates generally to digital information reproducing apparatuses such as optical disk apparatuses, and more specifically, to a digital information reproducing apparatus for returning compressed digital information to its original size for reproduction.
2. Description of the Background Art
An apparatus having a function of reproducing compressed stored data such as mini disk apparatus includes a circuit for performing an expansion processing to return compressed data to its original size at the time of reproduction. The circuit performs such an expansion processing at a speed lower than reading of compressed data by the head. Therefore, a semiconductor memory for absorbing the difference in the data transfer rates is provided between the head and the circuit for performing expansion processing in the above-described apparatus.
A mini disk apparatus is for example provided with a DRAM as semiconductor memory, and part of the memory capacity of the DRAM is allocated for absorbing the difference in the data transfer rates at the time of reproduction. The remaining memory capacity of the DRAM is allocated for data protection at the time of disturbance such as oscillation or when a track jump for moving the head to a target track is performed. Therefore, if data reading is temporarily stopped for such an oscillation or track jump, data is held in the DRAM for a certain period of time taking advantage of the difference in the data transfer rates. Accordingly, if the head becomes ready for normal data reading during this period, reproduction will not be interrupted.
In a conventional mini disk apparatus, a semiconductor memory is utilized both for absorbing the difference in the data transfer rates and for data protection. If the moving distance of the head is prolonged when it performs a track jump, time required for the moving is prolonged as well, and sufficient memory capacity for data protection can not be secured in some cases. In order to normally reproduce data at the time of track jump, a series of search operations for moving the optical pickup to a target track and causing emitted light to follow the track must be performed at a high speed.
In order to perform the search operations at a high speed, circuitry of control system such as a servo circuit must be designed to increase the speed of optical pickup feeding control or the like, which was not easy to achieve. Also for high speed searching operations, the driving system must supply an increased amount of electric power, and power consumption increases as a result.